I read a 500 page novel in a day. If it's slow going, I might have to finish it the next morning. According to your little test, I read a 273 wpm, with a 82% comprehension rate. I think that the comprehension rate is higher when I read novels because that test sequence was boring, and because I rarely forget much of anything that's happened in a book, even if I read it years ago.
This is exactly why I try to find books that are almost 1000 pages long, and that are part of a series, that way I can be entertained for a while. Still, it never takes me long, I read a book series that was over 10,000 pages long in less than 2 weeks.
To answer your question, yes this happens because I sit down to read the book for hours at a time and actually read the book for that entire time(most of it, I occasionally stop to think about the book). If this isn't just some bad joke, and you really read like that, you either need to find a better book, or you need to go get checked out for ADD or ADHD.